Long Length Electrodes

ABSTRACT

An embodiment disclosed herein includes a monolithic graphite electrode. The electrode has a main body having a length of more than 3050 mm. Another embodiment disclosed herein includes an electrode column comprising a plurality of monolithic graphite electrodes. The column has a length of more than 3050 mm of electrode per joint. A further embodiment discussed herein is the practice of increasing the length of the electrode to minimize the occurrence of an electrode joint in the electrode column for a given length. This practice will improve efficiencies for both electrode manufacturers as well as electric arc furnace operators.

BACKGROUND OF THE INVENTION

The present application is a Continuation of copending U.S. patentapplication Ser. No. 12/062,005, filed on Apr. 3, 2008, entitled LONGLENGTH ELECTRODE, which claims priority to U.S. Provisional PatentApplication No. 60/922,519, filed on Apr. 9, 2007, also entitled LONGLENGTH ELECTRODES which is hereby incorporated by reference in itsentirety.

TECHNICAL FIELD

The present application relates to graphite articles, and a process forpreparing the graphite articles. More particularly, the inventionconcerns articles such as graphite electrodes.

BACKGROUND ART

Graphite electrodes are used in the steel industry to melt the metalsand other ingredients used to form steel in electrothermal furnaces. Theheat needed to melt metals is generated by passing current through aplurality of electrodes, usually three, and forming an arc between theelectrodes and the metal. Electrical currents in excess of 50,000amperes are often used. The resulting high temperature melts the metalsand other ingredients. Generally, the electrodes used in steel furnaceseach consist of electrode columns, that is, a series of individualelectrodes joined to form a single column. In this way, as electrodesare depleted during the thermal process, replacement electrodes can bejoined to the column to maintain the length of the column extending intothe furnace.

Generally, electrodes are joined into columns via a pin (sometimesreferred to as a nipple) that functions to join the ends of adjoiningelectrodes. Typically, the pin takes the form of opposed male threadedsections, with at least one end of the electrodes comprising femalethreaded sections capable of mating with the male threaded section ofthe pin. Thus, when each of the opposing male threaded sections of a pinare threaded into female threaded sections in the ends of twoelectrodes, those electrodes become joined into an electrode column.Commonly, the joined ends of the adjoining electrodes and the pin therebetween, is referred to in the art as a pin joint.

Given the extreme thermal stress that the electrode and the joint (andindeed the electrode column as a whole) undergoes, mechanical/thermalfactors such as strength, thermal expansion, and crack resistance mustbe carefully balanced to avoid damage or destruction of the electrodecolumn or individual electrodes. For instance, longitudinal (i.e., alongthe length of the electrode/electrode column) thermal expansion of theelectrodes, especially at a rate different than that of the pin, canforce the joint apart, reducing effectiveness of the electrode column inconducting the electrical current. Typically, the across graincoefficient of thermal expansion (CTE) of the pin is higher than theacross the grain CTE of the electrode. Therefore transverse (i.e.,across the diameter of the electrode/electrode column) thermal expansionof the pin being somewhat greater than that of the electrode may be usedto form a firm connection between pin and electrode; however, if thetransverse thermal expansion of the pin greatly exceeds that of theelectrode, damage to the electrode or separation of the joint mayresult. Again, this can result in reduced effectiveness of the electrodecolumn or even destruction of the column if the damage is so severe thatthe electrode column fails at the joint section.

As a consequence of the above, the pin joint is a point of concern in anelectrode column. To improve the reliability of pin joints, pins areoften made from graphite of higher density and strength than theelectrode itself. However, increasing the strength and density ofgraphite pins also increases the manufacturing time and cost of the pin,and hence the cost of the electrode column formed using pin joints.There have been other efforts to improve the reliability of the pinjoint. For example, an electrode pin joint may include a reservoir tohold a quantity of pitch binder as a curable binder. While on thefurnace, the pitch will reach its softening point and will flow betweenthe threads. Upon more intense heating, the pitch will carbonize inbetween the threads and hold the adjacent threads together. Variationson this concept include the pin having one or more flow channels and/orthe pin joint including more than one pitch reservoir or the location ofthe reservoir being varied.

In the past, efforts have also been taken to eliminate the pin from thejoint in order to improve the performance of the electrode columnsystem. Prior attempts to eliminate the pin, which have been attempted,include a threaded electrode end or other electrode mating means beingemployed. For example electrodes have been made which include anintegral threaded tang at one end of the electrode, also known as apinless joint. Industry acceptance of a pinless joint has lagged,however, since the strength of the graphite in the electrode is viewedby some as not sufficient to maintain the integrity of the electrodecolumn. For the above reasons and others, the joint between two adjacentelectrodes in an electrode column is an area of concern for an operatorof an electric arc furnace.

A Soderberg Paste electrode is an example of a prior attempt to producea pinless electrode. The Soderberg electrode is a continuously formedelectrode used in an electric arc furnace, in which a mixture ofpetroleum coke and coal-tar pitch is continuously added to a steelcasing and is baked as it passes through the heated casing, such thatthe baked electrode emerging into the furnace continuously replaces theelectrode being consumed. Since these electrodes are baked and notgraphitized, their performance is not suitable for use in electric arcsteelmaking. The paste electrodes are typically used in arc furnaces formanufacturing ferroalloys, aluminum, nickel, copper and othernon-ferrous applications.

In light of the above issues, electrode joint designs have beenstandardized over the years. These standards specify the height anddiameter designs for pins along with the parameters for the threads ofthe socket of an electrode. In addition to standards regarding theelectrode joint, standards have also been drafted and approved regardingthe length and diameter of the electrode. Examples of one such standardare IEC 60239 and JIS R7201. In each one of these standards the lengthof the electrode varies from no more than 2900 mm to about 825 mm andthe diameter of the electrode may vary from between 765 mm to 352 mm foran electrode of 2900 mm to 2275 mm in length.

Another issue for a steel manufacturer is downtime and other problemsassociated with electrode additions to the arc furnace. Each timeanother electrode is to be added to an electrode column or a new columnis to be added to the furnace, the furnace must be shut down while theelectrode or electrode column is added. Typically, for a furnace wherethree electrode columns are in simultaneous operation, the equivalent ofone electrode will be consumed over the course of about one eight (8)hour shift. Thus, to add an electrode to a column, or to exchange ashortened column with one of longer length, the furnace must be shutdown about three times during every twenty-four (24) hour period.

An example of how electrode columns are installed on a furnace isillustrated in FIGS. 3 and 4. FIG. 3 is a top view of the electric arcfurnace depicted in FIG. 4. As illustrated, the three electrode columns104, 120, and 130 are installed in furnace 102. Typically a furnaceoperated on alternating electric current will have three such columns,where a furnace operating on direct electrical current will use largerdiameter electrodes in a single electrode column.

When a particular electrode column is consumed, typically the electricalcurrent to create the arc to reclaim the steel is turned off and theremainder of the consumed column is removed from the furnace. The poweris then turned on and the current is transmitted through one or more ofthe remaining electrode columns and/or replacement column. Depicted inFIG. 4 is a view of electrical arc furnace 102 which shows two (2)electrode columns 104 and 120. Column 104 includes three (3) electrodes106, 108, and 110. The joints between the electrodes of column 104 arerepresented as reference numerals 112 and 114.

Electrode column 120 includes two electrodes 122 and 124. In thedepicted example, an electrode, such as electrode 110 may be added toelectrode column 104 by the use of an electrode robot 126. As shownrobot 126 is used to add a third electrode to a column alreadycomprising more than one electrode. Robot 126 may be used to align androtate the electrode being added to the column to engage a threadedportion of the top joint element of the electrode directly below theelectrode being added. Robot 126 may travel along rails 128, shown inFIG. 4 or may be positioned over the column by the use of an overheadcrane.

Similar to what was previously discussed, when an electrode is beingadded to a column, the electrical current being passed through a columnof the electrodes in furnace 102 is turned off and the significantproduction time is lost due to this change.

One method of reducing electrode additions at the furnace is to join tworelatively shorter electrodes together prior to delivery to thesteelmaker, as described in published U.S. patent application2006/0140244. However, this approach has the disadvantage that each ofthe shorter electrodes must be machined to have its own threaded tangand socket portions prior to assembly, requiring the machining of fourthreaded sections instead of two for a single electrode. The need tomachine four threaded sections requires additional labor and time, andwastes the high value graphite material that is machined away to makethe threaded section. Thus, there is a need for a monolithic electrode,that is, an electrode without an added joint that can also provide theuser with a longer period of productivity between electrode additions.

BRIEF DESCRIPTION

The present invention seeks to provide a monolithic graphite electrodehaving advantages over known such electrodes.

According to the present invention there is provided a monolithicgraphite electrode comprising a main body having a length of more than3050 mm.

Advantageously, the electrode of the present invention overcomesproblems with standard type electrodes such as furnace downtime.

Preferably, the electrode main body includes a pair of end faces, eachface includes a socket.

Preferably, the length of the main body comprises more than 3330 mm.Preferably still, the length of the main body comprises more than 3430mm. More preferably, the length of the main body comprises more than3680 mm.

Preferably, a diameter of the electrode comprises from about 500 mm toabout 900 mm. More preferably, the diameter of the electrode comprisesfrom about 500 mm to 860 mm, even more preferably no more than 850 mm.

Preferably, in one embodiment, the threads per inch (‘TPI”) of the tangcomprises less then four (4), e.g., three (3) or two (2) and a TPI ofthe socket comprises two. Preferably a taper of the tang comprises 9° orgreater.

Another embodiment disclosed herein includes an electrode columncomprising a plurality of monolithic graphite electrodes. The column hasa length of more than 3050 mm of electrode per joint and more preferably3300 mm or more per electrode joint.

Preferably, the column has an overall length of at least 6350 mm andless than two joints.

A further embodiment discussed herein is the practice of increasing thelength of the electrode to minimize the occurrence of an electrode jointin the electrode column for a given length. This practice will improveefficiencies for both electrode manufacturers as well as electric arcfurnace operators.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in more detail, by way ofexample only with reference to the accompanying drawings, in which:

FIG. 1 is a view of a pin-socket electrode;

FIG. 2 is a view of a pinless joint electrode;

FIG. 3 is a top schematic view of furnace shown in FIG. 4; and

FIG. 4 is a front view of an electrode column on an electric arcfurnace.

DETAILED DESCRIPTION

As noted above, graphite articles (graphite articles is used herein toinclude at least graphite electrodes) could be fabricated by firstcombining a particulate fraction comprising calcined coke (when thegraphite article to be produced is graphite electrode), pitch andmesosphere pitch or PAN-based carbon fibers into a stock blend. Morespecifically, crushed, sized and milled calcined petroleum coke is mixedwith a coal-tar pitch binder to form the blend. The particle size of thecalcined coke is selected according to the end use of the article, andis within the skill in the art. Generally, in graphite electrodes foruse in processing steel, particles up to about 25 millimeters (mm) inaverage diameter are employed in the blend. The particulate fractionpreferably includes a small particle size filler comprising coke powder.Other additives that may be incorporated into the small particle sizefiller include iron oxides to inhibit puffing (caused by release ofsulfur from its bond with carbon inside the coke particles), coke powderand oils or other lubricants to facilitate extrusion of the blend.

The blend may also include mesophase pitch-based carbon fibers or fibersderived from PAN (polyacrylonitrile), added after mixing of the stockhas already begun. The fibers used should advantageously have a Young'smodulus (after carbonization) of about 100 GPa to about 275 GPa orhigher (Cytec's Thornel T-300 PAN fibers have a tensile modulus of 231GPahttp://www.cytec.com/business/engineeredmaterials/CFInternet/cfThornelT-300PAN.shtm).The fibers preferably have an average diameter of about 6 to about 15microns (T-300 is 7 micron), a tensile strength of about 1.4 GPa toabout 2.8 GPa. In certain embodiments, the tensile strength of thefibers may be as high as up to 5 GPa, (The tensile strength of T-300 is3.75 GPa). Preferably the length of the fibers is about 4 mm to about 32mm in length on average. Suitable lengths of fiber include an averagelength of about 6 mm or less, about 12 mm or less, about 18 mm or less,or about 25 mm or less. It is also preferred that the carbon fibers arenot longer than the biggest coke particle. Most advantageously, thefibers are added to the blend as bundles containing between about 2000and about 20,000 fibers per bundle, compacted with the use of a sizing(U.S. Pat. No. 6,916,435).

As noted, the carbon fibers to be included in the blend are based onmesophase pitch or PAN. Mesophase pitch fibers are produced from pitchthat has been at least partially transformed to a liquid crystal, orso-called mesophase, state. Mesophase pitch can be prepared fromfeedstocks such as heavy aromatic petroleum streams, ethylene crackertars, coal derivatives, petroleum thermal tars, fluid cracker residuesand pressure treated aromatic distillates having a boiling range from340° C. to about 525° C. The production of mesophase pitch is describedin, for example, U.S. Pat. No. 4,017,327 to Lewis et al. Typically,mesophase pitch is formed by heating the feedstock in a chemically inertatmosphere (such as nitrogen, argon, helium or the like) to atemperature of about 350° C. to 500 ° C. A chemically inert gas can bebubbled through the feedstock during heating to facilitate the formationof mesophase pitch. For preparation of carbon fibers, the mesophasepitch should have a softening point, that is, the point at which themesophase pitch begins to deform, of less than about 400° C. and usuallyless than about 350° C. If the pitch has a higher softening point,formation of carbon fibers having the desired physical properties isdifficult.

Once the mesophase pitch is prepared, it is spun into filaments of thedesired diameter, by known processes such as by melt spinning,centrifugal spinning, blow spinning or other processes which will befamiliar to the skilled artisan. Spinning produces carbon fiberssuitable for use in preparing the electrode of the present invention.The filaments are then thermoset at a temperature no higher than thesoftening point of the pitch (but usually above 250° C.) for about 5 to60 minutes, then further treated at extremely high temperatures, on theorder of up to about 1000° C. and higher, and in some cases as high asabout 3000° C., more typically about 1500° C. to 1700° C., to carbonizethe fibers. The carbonization process takes place in an inertatmosphere, such as argon gas, for at least about 0.5 minutes. Mostcommonly, carbonization uses residence times of between about 1 and 25minutes. The fibers are then cut to length and formed into bundles. Suchfibers, bundled as described, are commercially available from, forinstance, Cytec Industries Inc. of West Paterson, N.J. and MitsubishiChemical Functional Products Inc. of Tokyo, Japan.

One method of making the PAN fibers comprises spinning the fibers from asolution of polyacrylonitrile. The fibers are then stabilized in thesame manner as are the mesophase pitch-based fibers. The production ofPAN fibers is described, for instance, by Dan D. Edie and John J. McHughin High Performance Carbon Fibers at pages 119-138 of Carbon Materialsfor Advanced Technologies, 1st Ed., Elsevier Science Ltd. 1999, thedisclosure of which is incorporated herein by reference in its entirety.

The carbon fibers are preferably included in the stock blend at a levelof about 0.5 to about 6 parts by weight of carbon fibers per 100 partsby weight of calcined coke. Most preferably, the fibers are present at alevel of about 1.25 to about 6 parts by weight fibers per 100 parts byweight of coke. In terms of the blend as a whole (excluding binder), thecarbon fibers are incorporated at a level of about 1% to about 5.5% byweight, more preferably about 1.5% to up to about 5.5%, even morepreferably, about 5.0% or less.

After the blend of particulate fraction, pitch binder, carbon fibers,etc. is prepared, the body is formed (or shaped) by extrusion though adie or molded in conventional forming molds to form what is referred toas a green stock. The forming, whether through extrusion or molding, isconducted at a temperature close to the softening point of the pitch,usually about 100° C. or higher. Although the die or mold can form thearticle in substantially final form and size, machining of the finishedarticle is usually needed, at the very least to provide structure suchas threads. The size of the green stock can vary; for electrodes thediameter can vary between about 220 mm and 850 mm.

After extrusion, the green stock is heat treated by baking at atemperature of between about 700° C. and about 1100° C., more preferablybetween about 800° C. and about 1000° C., to carbonize the pitch binderto solid pitch coke, to give the article permanency of form, highmechanical strength, good thermal conductivity, and comparatively lowelectrical resistance, and thus form a carbonized stock. The green stockis baked in the relative absence of air to avoid oxidation. Bakingshould be carried out at a rate of about 1° C. to about 5° C. rise perhour to the final temperature. After baking, the carbonized stock may beimpregnated one or more times with coal tar or petroleum pitch, or othertypes of pitches or resins known in the industry, to deposit additionalcoke in any open pores of the stock. Each impregnation is then followedby an additional baking step.

After baking, the carbonized stock is then graphitized. Graphitizationis by heat treatment at a final temperature of between about 2500° C. toabout 3400° C. for a time sufficient to cause the carbon atoms in thecoke and pitch coke binder to transform from a poorly ordered state intothe crystalline structure of graphite. Advantageously, graphitization isperformed by maintaining the carbonized stock at a temperature of atleast about 2700° C., and more advantageously at a temperature ofbetween about 2700 ° C. and about 3200° C. At these high temperatures,elements other than carbon are volatilized and escape as vapors. Thetime required for maintenance at the graphitization temperature usingthe process of the present invention is no more than about 18 hours,indeed, no more than about 12 hours. Preferably, graphitization is forabout 1.5 to about 8 hours.

As noted, once graphitization is completed; the finished article can becut to size and then machined or otherwise formed into its finalconfiguration. The finished article may be machined into a pin-socketelectrode as illustrated in FIG. 1, depicted as 10. As shown, electrode10 includes a main body (extending from end face to end face ofelectrode 10) 12, and pair of end faces 14 at each longitudinal end ofbody 12. A socket 16 may be machined into each end face 14, preferablysocket 16 includes threads 18. Preferably main body 12 of electrode 10has a length of more than 3050 mm (120 inches), more preferably 3300 mm(130 inches) or more, even more preferably 3550 mm (140 inches) or more,and most preferably 3680 mm (145 inches) or more. In one particularexample, main body 12 has a length of greater than 3800 mm (about 150inches). Due to the green stock losing some length during thegraphitization and machining steps, electrode 10 is preferably formedfrom a green body having an electrode length of 3200 mm (126 inches) ormore, more preferably 3430 mm (135 inches) or more, and even morepreferably 3810 mm (150 inches) or more.

Shown in FIG. 2 is an electrode 20 which includes pinless jointtechnology. Electrode 20 also includes a main body (end face to end oftang) 22 and further includes a socket 26 in an end face 24 at onelongitudinal end of body 22. Electrode 20 may also include a threadedtang 28 at or about a second longitudinal end of body 22. Body 22 ofelectrode 20 may have a length of at least 2920 mm (115 inches). In oneparticular embodiment, body 22 has a length of at least 3175 (125inches), preferably at least 3300 mm (130 inches), more preferably atleast 3425 mm (135 inches), and even more preferably at least 3550 mm(140 inches), and most preferably at least 3680 mm (145 inches). In onecertain embodiment, the length of body 22 is at least about 3800 mm(about 150 inches). One way to measure the overall length of electrode20 is from the exterior surface of end face 24 to the tip of tang 28.Examples of typical lengths of tang 28 are about 500 mm (20 inches) toabout 630 mm (25 inches), measured from the tip of the tang to a base ofthe tang, illustrated by line “L” on FIG. 2. Preferably tang 28 extendsfrom body 12 at a taper angle of “α” In one preferred embodiment, α isabout 9° or greater. In another embodiment α is about 15° or greater.Optionally, electrode 20 may include a seal around tang 28, not shown.

The diameter of the above described electrodes 10 and 20 may vary asdesired by the end user. The diameter of electrode 10 or 20 may varyfrom about 350 mm (14 inches) to about 860 mm (34 inches) as selected bythe end user. Also the thread pitch in sockets 16 as well as socket 26may vary as selected by the end user. The thread pitch or threads perinch (TPI) may vary from two (2) to eight (8) TPI for any socket ofelectrode 10 or 20. The threads 40 on tang 28 may have the same, or ifdesired different, pitch as the threads of socket 26. Similarly it istypical that both sockets 16 have the same TPI, however, if desiredsockets 16 may have different TPI. The same is true for socket 26 andtang 28 in that typically socket 26 will have the same TPI as tang 28 orvice versa. However, the TPI may vary between socket 26 and tang 28 ifdesired by the end user.

Preferably, the above electrode may be included in the electrode columnsuch that the column will include more than 3050 mm of length ofmonolithic electrode per joint between adjacent electrodes in theelectrode column; more preferably, the length comprises more than 3300mm. In one particular embodiment, the electrode column may comprise over6300 mm and less than two joints between the electrodes, which make upthe column.

An advantage of the disclosed embodiments is that they reduce thefrequency of the occurrence of the joint in the electrode column, thusincreasing the maximum length of electrode per joint. To the furnaceoperator, the disclosed subject matter will offer the advantage ofincreased yield of steel, less downtime per ton of steel reclaimed, anddecreased labor requirements associated with electrode consumption perton of steel reclaimed. For the electrode manufacturer, this is anopportunity to tailor electrodes to the specific requirements ofindividual steel manufacturers.

The various described embodiments may be practiced separately or in anycombination thereof.

What is claimed is:
 1. A monolithic graphite electrode comprising a mainbody having a length of at least 3175 mm and a pair of end faces,wherein each end face located at an opposed end of the body, wherein afirst end face includes a socket and a second end face includes athreaded tang.
 2. The electrode of claim 1 wherein the TPI of the tangcomprises less than four.
 3. The electrode of claim 2 wherein the TPIcomprises less than three.
 4. The electrode of claim 1 wherein diameterof the electrode comprises no more than 850 mm.
 5. The electrode ofclaim 1 wherein the diameter of the electrode comprises from 500 mm to900 mm.
 6. An electrode column wherein one of the electrodes in thecolumn comprises the electrode of claim
 1. 7. The electrode column ofclaim 6 wherein the column has an overall length of at least 6350 mm andless than two joints.
 8. The electrode column of claim 6 comprising over6300 mm of length and less than two joints.
 9. The electrode of claim 1wherein an angle of taper of the tang comprises at least 9°.
 10. Theelectrode of claim 9 wherein the angle of taper comprises at least 15°.